Etchless acoustic waveguiding in integrated acousto-optic waveguides

ABSTRACT

An acousto-optic waveguide device comprises a substrate comprising a first material having a first refractive index and a first acoustic velocity; a cladding layer over the substrate, the cladding layer comprising a second material having a second refractive index that is distinct from the first refractive index, the second material having a second acoustic velocity that is distinct from the first acoustic velocity; and an optical core surrounded by the cladding layer, the optical core comprising a third material having a third refractive index that is higher that the first refractive index and the second refractive index, the third material having a third acoustic velocity that is distinct from the first acoustic velocity and the second acoustic velocity. The cladding layer that surrounds the optical core has a thickness configured to substantially confine acoustic waves to the cladding layer when an optical signal propagates through the optical core.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication No. 62/397,068, filed on Sep. 20, 2016, which is hereinincorporated by reference.

This application is a divisional of U.S. application Ser. No.15/591,836, filed on May 10, 2017, which is herein incorporated byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under N66001-16-C-4017awarded by SPAWAR Systems Center Pacific. The Government has certainrights in the invention. This material is based upon work supported bythe Defense Advanced Research Projects Agency (DARPA) and Space andNaval Warfare Systems Center Pacific (SSC Pacific).

BACKGROUND

The simultaneous guiding of both optical and acoustic waves is anattractive capability for integrated platforms, because it allows forthe tunable enhancement of such interesting and advantageous effects asBrillouin scattering. However, present techniques to include acousticwaveguiding in optical platforms typically rely on the introduction ofair/solid material boundaries which, in addition to complicatingfabrication, increase scattering loss for both the optical and acousticwaves.

SUMMARY

An acousto-optic waveguide device comprises a substrate comprising afirst material having a first refractive index and a first acousticvelocity; a cladding layer over the substrate, the cladding layercomprising a second material having a second refractive index that isdistinct from the first refractive index, the second material having asecond acoustic velocity that is distinct from the first acousticvelocity; and an optical core surrounded by the cladding layer, theoptical core comprising a third material having a third refractive indexthat is higher that the first refractive index and the second refractiveindex, the third material having a third acoustic velocity that isdistinct from the first acoustic velocity and the second acousticvelocity. The cladding layer that surrounds the optical core has athickness configured to substantially confine acoustic waves to thecladding layer when an optical signal propagates through the opticalcore.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the present invention will become apparent to those skilledin the art from the following description with reference to thedrawings. Understanding that the drawings depict only typicalembodiments and are not therefore to be considered limiting in scope,the invention will be described with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIGS. 1A and 1B are cross-sectional end views of an acousto-opticwaveguide device, according to one embodiment;

FIGS. 2A-2F are end views showing an exemplary method of fabricating anacousto-optic waveguide device;

FIG. 3 is a cross-sectional side view of an acousto-optic waveguidedevice, according to an exemplary embodiment;

FIG. 4 is a graph of a simulated gain coefficient for the acousto-opticwaveguide device of FIG. 3; and

FIG. 5 is a perspective view of an integrated fiber optic gyroscope thatcan employ an acousto-optic waveguide, according to an exemplaryembodiment.

DETAILED DESCRIPTION

In the following detailed description, embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. It is to be understood that other embodiments may be utilizedwithout departing from the scope of the invention. The followingdetailed description is, therefore, not to be taken in a limiting sense.

Integrated acousto-optic waveguide devices are provided that areconfigured to have etchless acoustic waveguiding features. The waveguidedevices are fabricated in such a way that acoustic confinement does notrely on a material etch, but instead makes use of acoustic reflection atinterfaces between two solids with dissimilar acoustic velocities.

In comparison to prior techniques, the present approach achievesacoustic waveguiding by surrounding an optical waveguide section with amaterial possessing an acoustic velocity (or speed of sound through thematerial) that is distinct from the rest of the materials in the opticalwaveguide section. In addition to providing a waveguide device that isreadily fabricated, the present technique has a negligible effect on theproperties of the optical core of the waveguide device.

Furthermore, the low velocity contrast used to achieve acousticwaveguiding yields a low acoustic loss coefficient, providing a furtherenhancement to effects which rely on the co-propagation of the opticaland acoustic waves. Lateral acoustic confinement is not provided by thepresent technique, but, if the lateral area of the optical mode issufficiently large (e.g., about 3-5 microns), the acoustic wave willhave minimal divergence in the lateral direction, and thus confinementalong that direction will not be necessary.

During operation of the acousto-optic waveguide device, as lightpropagates through the waveguide device the light generates acousticwaves through electrostriction. By providing a waveguiding materialhaving a sufficiently large elasto-optic effect, the present waveguidedevice has Brillouin scattering at a rate significantly larger than thatof waveguides that which do not possess the acoustic confinementintroduced through the present technique.

Further details of the present waveguide device and a method forfabrication are described hereafter with reference to the drawings.

FIGS. 1A and 1B illustrate an acousto-optic waveguide device 100,according to one embodiment. The waveguide device 100 generally includesa substrate 110, a cladding layer 120 over substrate 110, and an opticalcore 130 embedded in and surrounded by cladding layer 120. An optionaltop layer 140 may be over cladding layer 120.

FIG. 1A depicts the optical waveguide material properties of waveguidedevice 100. The substrate 110 is composed of a first material having afirst refractive index (n₁), and cladding layer 120 is composed of asecond material having a second refractive index (n₂) that is distinctfrom the first refractive index. The optical core 130 is composed of athird material having a third refractive index (n₃) that is greater thanboth the first refractive index and the second refractive index, suchthat an optical signal will propagate through optical core 130. Theoptional top layer 140 may be composed of the same first material assubstrate 110 having the first refractive index.

FIG. 1B depicts the acoustic waveguide material properties of waveguidedevice 100. The first material of substrate 110 has a first acousticvelocity (v₁), and the second material of cladding layer 120 has asecond acoustic velocity (v₂) that is distinct from the first acousticvelocity. The third material of optical core 130 has a third acousticvelocity (v₃) that is distinct from the second acoustic velocity of thesecond material and the first acoustic velocity of the first material.When the optional top layer 140 is composed of the same first materialas substrate 110, the material of top layer 140 also has the firstacoustic velocity.

The optical core 130 can be composed of various materials, such assilicon, silicon nitride (SiNx), silicon oxynitride (SiON), siliconcarbide (SiC), diamond, silicon germanium (SiGe), germanium, galliumarsenide (GaAs), gallium nitride (GaN), gallium phosphide (GaP), lithiumniobate (LiNbO₃), or combinations thereof. The optical core 130 can beformed to have a thickness of about 20 nm to about 100 nm, for example.

The cladding layer 120 can be composed of various materials, such assilicon dioxide (SiO₂), silicon oxynitride, zinc oxide (ZnO), aluminumoxide (Al₂O₃), calcium fluoride (CaF₂), or combinations thereof. Thecladding layer 120 can be formed to have a thickness of about 4 μm toabout 10 μm, for example.

The substrate 110 can be composed of any wafer material that isatomically flat, such as any of the above materials. The optional toplayer 140 can also be composed of any of the above materials.

The substrate 110, cladding layer 120, and optional top layer 140 can beformed of the same material and thus have the same chemical composition,but in this case are tailored to have differing properties depending onthe fabrication methods employed. For example, substrate 110 and toplayer 140 can be formed with a thermal oxide such as thermal SiO₂, whichhas a first density, and cladding layer 120 can be formed with a plasmaenhanced chemical vapor deposition (PECVD) oxide such as PECVD SiO₂,which has a second density different from the first density of thethermal SiO₂. Other properties that can be tailored include fabricationof the respective layer materials to be amorphous and crystalline.Alternative methods for fabricating the layer materials includesputtering, low pressure CVD, atomic layer deposition, or the like.

FIGS. 2A-2F depict an exemplary method of fabricating an acousto-opticwaveguide device. Fabrication of the waveguide device begins with awafer substrate 210 having an upper surface 212, as shown in FIG. 2A.The wafer substrate 210 can be formed of a first material comprising,for example, a thermal oxide as its topmost layer, with the firstmaterial having a first refractive index and a first acoustic velocity.

Next, an initial amount of a second material is deposited over uppersurface 212 of wafer substrate 210 to form a partial cladding layer 220,as shown in FIG. 2B. The second material has a second refractive indexthat is distinct from the first refractive index of the first material.The second material also has a second acoustic velocity that is distinctfrom the first acoustic velocity of the first material. For example,partial cladding layer 220 can be formed by depositing about 2-4 micronsof an acoustically dense dielectric material, such as PECVD-depositedsilicon dioxide, which has a higher acoustic velocity than thermaloxide.

Thereafter, a third material is deposited over partial cladding layer220 to form an optical layer 230, as depicted in FIG. 2C. The thirdmaterial has a third refractive index that is greater than the firstrefractive index of the first material and the second refractive indexof the second material. The third material also has a third acousticvelocity that is distinct from the first acoustic velocity of the firstmaterial and the second acoustic velocity of the second material. Forexample, optical layer 230 can be formed by depositing about 20-100 nmof a dielectric material with a refractive index higher than any of theother constituent materials. In one implementation, the dielectricmaterial of optical layer 230 is PECVD-deposited silicon nitride.

As shown in FIG. 2D, an optical core structure 234 is formed by removingportions of the third material of optical layer 230 to expose portionsof partial cladding layer 220, with optical core structure 234comprising the remaining third material. For example, optical corestructure 234 can be formed by etching optical layer 230, such asthrough conventional electron-beam lithography or photolithography-basedprocedures.

An additional amount of the second material is then deposited overoptical core structure 234 and the exposed portions of partial claddinglayer 220 to form a full cladding layer 224 that surrounds optical corestructure 234, as depicted in FIG. 2E. For example, the additionalamount of the second material can be about 2-4 microns of theacoustically dense dielectric material, such as PECVD-deposited silicondioxide.

In an optional step shown in FIG. 2F, a top layer 240 can be formed overfull cladding layer 224. The top layer 240 can be formed by depositingthe same first material used to form substrate 210, such as the thermaloxide having a lower acoustic velocity. The top layer 240 is optionalsince it is not necessary for acoustic confinement of the acousto-opticwaveguide device.

FIG. 3 illustrates an acousto-optic waveguide device 300, according toan exemplary embodiment, which can be fabricated as describedpreviously. The waveguide device 300 generally includes a substrate 310,which can be composed of thermal SiO₂, for example. A cladding layer 320is over substrate 310, and an optical core 330 is embedded in claddinglayer 320. The cladding layer 320 can be composed of PECVD-depositedSiO₂, for example, and has a given thickness (T) such as about 4 μm toabout 10 μm. The optical core 330 can be composed of silicon nitride(SiN_(x)), for example. An optional top layer 340 may be over claddinglayer 320 and can be composed of thermal SiO₂, for example. Asillustrated in FIG. 3, during operation of waveguide device 300, light(L) is coupled into an edge 302, and as the light propagates throughoptical core 330, the light will generate acoustic waves throughelectrostriction. If the waveguiding material has a sufficiently largeelasto-optic effect, this will give rise to Brillouin scattering at asignificantly larger rate.

FIG. 4 is a graph of a simulated gain coefficient for acousto-opticwaveguide device 300 (FIG. 3). The graph of FIG. 4 shows plots ofBrillouin gain with respect to frequency for different thicknesses (T)of cladding layer 320 composed of a PECVD oxide. As shown in FIG. 4, theBrillouin gain increases with respect to frequency as the thickness ofthe PECVD oxide decreases from 10 μm down to 4 μm.

The acousto-optic waveguide device disclosed herein may be used, forexample, in an integrated photonics circuit, in either a straightwaveguide or a resonator, to couple energy from a forward propagatingpump wave into a counter-propagating Stokes wave. This process may becascaded multiple times, corresponding to the generation of higher-orderStokes waves propagating in alternating directions. The Stokes waves mayact as carriers for data encoded in the optical regime, may serve tomonitor the Sagnac effect in optical gyroscopes, or may monitor thetemperature and stress in the constituent integrated photonics circuit.

FIG. 5 illustrates an example of an integrated fiber optic gyroscope500, which can employ the acousto-optic waveguide device. The fiberoptic gyroscope 500 includes an integrated photonics circuit or chip510, which is in optical communication with an input optical fiber 520and an output optical fiber 530. The input optical fiber 520 directs alight beam from a source to an acousto-optic waveguide 540 in chip 510.Counter-propagating light beams are generated in one or more ringresonators 550 coupled to acousto-optic waveguide 540 in chip 510. Thebeat frequencies of the counter-propagating light beams are used todetermine the rate of rotation based on output optical signals receivedby output optical fiber 530.

Example Embodiments

Example 1 includes an acousto-optic waveguide device comprising: asubstrate comprising a first material having a first refractive indexand a first acoustic velocity; a cladding layer over the substrate, thecladding layer comprising a second material having a second refractiveindex that is distinct from the first refractive index, the secondmaterial having a second acoustic velocity that is distinct from thefirst acoustic velocity; and an optical core surrounded by the claddinglayer, the optical core comprising a third material having a thirdrefractive index that is higher that the first refractive index and thesecond refractive index, the third material having a third acousticvelocity that is distinct from the first acoustic velocity and thesecond acoustic velocity; wherein the cladding layer that surrounds theoptical core has a thickness configured to substantially confineacoustic waves to the cladding layer when an optical signal propagatesthrough the optical core.

Example 2 includes the acousto-optic waveguide device of Example 1,further comprising a top layer over the cladding layer.

Example 3 includes the acousto-optic waveguide device of Example 2,wherein the top layer comprises the first material.

Example 4 includes the acousto-optic waveguide device of any of Examples1-3, wherein the first and second materials have the same chemicalcomposition, but have different densities.

Example 5 includes the acousto-optic waveguide device of any of Examples1-4, wherein the second material comprises silicon dioxide, siliconoxynitride, zinc oxide, aluminum oxide, calcium fluoride, orcombinations thereof.

Example 6 includes the acousto-optic waveguide device of any of Examples1-5, wherein the third material comprises silicon, silicon nitride,silicon oxynitride, silicon carbide, diamond, silicon germanium,germanium, gallium arsenide, gallium nitride, gallium phosphide, lithiumniobate, or combinations thereof.

Example 7 includes the acousto-optic waveguide device of any of Examples1-6, wherein the waveguide device is implemented in an integratedphotonics circuit or chip.

Example 8 includes the acousto-optic waveguide device of Example 7,wherein the integrated photonics circuit or chip is part of a fiberoptic gyroscope.

Example 9 includes a method of fabricating an acousto-optic waveguidedevice, the method comprising: providing a wafer substrate having anupper surface, the wafer substrate comprising a first material having afirst refractive index and a first acoustic velocity; depositing aninitial amount of a second material over the upper surface of the wafersubstrate to form a partial cladding layer, the second material having asecond refractive index that is distinct from the first refractiveindex, the second material having a second acoustic velocity that isdistinct from the first acoustic velocity; depositing a third materialover the partial cladding layer to form an optical layer, the thirdmaterial having a third refractive index that is greater than the firstrefractive index and the second refractive index, the third materialhaving a third acoustic velocity that is distinct from the firstacoustic velocity and the second acoustic velocity; removing portions ofthe third material of the optical layer to expose portions of thepartial cladding layer and form an optical core structure comprising theremaining third material; and depositing an additional amount of thesecond material over the optical core structure and the exposed portionsof the partial cladding layer to form a full cladding layer thatsurrounds the optical core structure.

Example 10 includes the method of Example 9, further comprising forminga top layer over the full cladding layer.

Example 11 includes the method of Example 10, wherein the top layercomprises the first material.

Example 12 includes the method of any of Examples 9-11, wherein thefirst and second materials have the same chemical composition, but havedifferent densities.

Example 13 includes the method of any of Examples 9-12, wherein thesecond material comprises silicon dioxide, silicon oxynitride, zincoxide, aluminum oxide, calcium fluoride, or combinations thereof.

Example 14 includes the method of any of Examples 9-13, wherein thethird material comprises silicon, silicon nitride, silicon oxynitride,silicon carbide, diamond, silicon germanium, germanium, galliumarsenide, gallium nitride, gallium phosphide, lithium niobate, orcombinations thereof.

Example 15 includes the method of any of Examples 9-14, wherein thewafer substrate comprises a thermal oxide.

Example 16 includes the method of any of Examples 9-15, wherein thesecond material and the third material are deposited using a processcomprising plasma enhanced chemical vapor deposition (PECVD), lowpressure CVD, sputtering, or atomic layer deposition.

Example 17 includes the method of Example 16, wherein the secondmaterial comprises PECVD-deposited silicon dioxide, and the thirdmaterial comprises PECVD-deposited silicon nitride.

Example 18 includes the method of any of Examples 9-17, wherein theportions of the third material are removed by a process comprisingelectron-beam lithography or a photolithography-based procedure.

Example 19 includes the method of any of Examples 9-18, wherein the fullcladding layer is formed to have a thickness of about 4 μm to about 10μm; and the optical core structure is formed to have a thickness ofabout 20 nm to about 100 nm.

Example 20 includes the method of any of Examples 9-19, wherein theacousto-optic waveguide device is formed as part of an integratedphotonics circuit or chip.

The present invention may be embodied in other specific forms withoutdeparting from its essential characteristics. The described embodimentsare to be considered in all respects only as illustrative and notrestrictive. The scope of the invention is therefore indicated by theappended claims rather than by the foregoing description. All changesthat come within the meaning and range of equivalency of the claims areto be embraced within their scope.

What is claimed is:
 1. A method of fabricating an acousto-optic waveguide device, the method comprising: providing a wafer substrate having an upper surface, the wafer substrate comprising a first material having a first refractive index and a first acoustic velocity; depositing an initial amount of a second material over the upper surface of the wafer substrate to form a partial cladding layer, the second material having a second refractive index that is distinct from the first refractive index, the second material having a second acoustic velocity that is distinct from the first acoustic velocity; depositing a third material over the partial cladding layer to form an optical layer, the third material having a third refractive index that is greater than the first refractive index and the second refractive index, the third material having a third acoustic velocity that is distinct from the first acoustic velocity and the second acoustic velocity; removing portions of the third material of the optical layer to expose portions of the partial cladding layer and form an optical core structure comprising the remaining third material; and depositing an additional amount of the second material over the optical core structure and the exposed portions of the partial cladding layer to form a full cladding layer such that the optical core structure is embedded in and surrounded by the full cladding layer.
 2. The method of claim 1, further comprising forming a top layer over the full cladding layer.
 3. The method of claim 2, wherein the top layer comprises the first material.
 4. The method of claim 1, wherein the first and second materials have the same chemical composition, but have different densities.
 5. The method of claim 1, wherein the second material comprises silicon dioxide, silicon oxynitride, zinc oxide, aluminum oxide, calcium fluoride, or combinations thereof.
 6. The method of claim 1, wherein the third material comprises silicon, silicon nitride, silicon oxynitride, silicon carbide, diamond, silicon germanium, germanium, gallium arsenide, gallium nitride, gallium phosphide, lithium niobate, or combinations thereof.
 7. The method of claim 1, wherein the wafer substrate comprises a thermal oxide.
 8. The method of claim 1, wherein the second material and the third material are deposited using a process comprising plasma enhanced chemical vapor deposition (PECVD), low pressure CVD, sputtering, or atomic layer deposition.
 9. The method of claim 8, wherein the second material comprises PECVD-deposited silicon dioxide, and the third material comprises PECVD-deposited silicon nitride.
 10. The method of claim 1, wherein the portions of the third material are removed by a process comprising electron-beam lithography or a photolithography-based procedure.
 11. The method of claim 1, wherein: the full cladding layer is formed to have a thickness of about 4 μm to about 10 μm; and the optical core structure is formed to have a thickness of about 20 nm to about 100 nm.
 12. The method of claim 1, wherein the acousto-optic waveguide device is formed as part of an integrated photonics circuit or chip. 